A Field-Effect Transistor (abbreviated as FET) using an electric-conductive high polymer to simulate an Si transistor is known as an electric conductivity-controlled element which is responsive to electric signals for controlling the electric conductivity of an organic substance, and is discussed for instance by F. Ebisawa et al. in J. Appl. Phys., 54 (1983) 3255; A. Tsumura et al. in Chem. Lett., (1986) 863; A. Tsumura et al. in Appl. Phys. Lett., 49 (1986) 1210; H. Koezuka et al. in Synth. Met., 18 (1987) 699; A. Tsumura et al. in Synth. Met., 25 (1988) 11 etc.
Polymer transistors which are discussed in these publications are of the same structure as Metal-Oxide-Semiconductor (abbreviated as MOS) or Metal-Insulator-Semiconductor (abbreviated as MIS). These polymer transistors work like a MOSFET made of Si, and their operating principle is supposed to be as follows:
FIG. 2 illustrates the construction of a conventional polymer transistor. Now assume in this Figure that a conductive polymer semiconductor 11 is of P-type. When an electric potential which is negative with respect to a metal electrode (source) 12 is applied to a metal electrode (gate) 13, positive polarization charges appear on the interface between the semiconductor 11 and an insulator layer 15 to cause accumulation of positive holes on the semiconductor surface. Conversely a given positive potential is applied to the gate metal electrode 13, causing appearance of negative polarization charges on the interface between the semiconductor 11 and insulator layer 15 whereby said negative charges and the positive holes in the P-type semiconductor cancel each other, thus forming a depletion region. Electrons will be increasingly accumulated on the semiconductor surface as the positive potential increases, and an "n-invention layer" will finally be formed. If a potential gradient should appear along the semiconductor surface, positive holes or electrons (carriers) in the accumulated layer will be transferred therealong. The width of the passage (channel) through which these carriers travel will vary with a potential applied to the semiconductor surface. Thus, the width of the channel, that is, the conductivity of carriers between a metal electrode (drain) 14 and the metal electrode (source) 12 can be controlled by controlling the potential difference or voltage between the metal electrode (source) 12 and the metal electrode (gate) 13.
An example of another conventional polymer transistor is an FET using ionization conduction in a solid electrolyte which consists of a high polymeric organic substance. This type of polymer transistor is discussed by Shuchi Chao et al, J. Am. Chem. Soc. 109,6627-6631, 1987.
At any rate, a conventional FET type device cannot control potentials to be applied to its gate, drain and source electrodes independently or separately, and therefore, it cannot be used for instance as a relay element.